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Comment by jfager | Hacker News Reader

It seems like there are a few climbers on here, could one of you please explain how a climber goes about deciding that a flake is too dangerous to apply a horizontal force to but safe enough to apply a vertical force to?

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Expanding flakes (ones that will move when you pull outward on them) are not good placements for active gear. As the cam pushes outward on a fall the flake might expand and the the cam might pull out because it is past it's usable range. Hammering pitons in a flake like this will do it too. As you hammer a piton higher up the flake it loosens the lower piton because it expands the flake from above to the point that the lower piton might fall out.

Thanks. I do understand the mechanics, but the article seemed to imply that there are some flakes that people do use traditional cams to climb. I was more curious in how the line between safe and unsafe gets drawn than the theory behind why horizontal forces on flakes are dangerous. In other words: you pull on flake, and it moves - obviously, horizontal force is unsafe. But: how do you then decide a vertical force on the same flake is safe? Is it just feeling and experience, or are there actual rules people generally follow?

Well from what I know this "anti-cam" device is the only rock protection that would not have a horizontal force component This guy is the first person to ever try it. All the other pro I can think of catches a fall by wedging into the crack, either by expanding or by taking advantage of a natural choke point.

Well, you can't actually do an engineering analysis of a rock face, so you're never going to achieve that level of rigor. Placing protections and building anchors is a combination of experience and intuition; observation and analysis techniques; and a bit of actual physics around angles, direction of force, rope stretch, and force multipliers.

There are guidelines that people follow but there is no hard and fast put this here like this and it will hold a fall from 75' up rules. It is a lot of feeling and intuition and most importantly redundancy!

You can peel a bandaid off, but you can't drag it along the plane of your skin. Replace the bandaid with a rock flake, the skin with a cliff, and the adhesive with ... best not think about it.

In the jargon, "Pull down, not out!"

There's a mechanical advantage issue too. The article discusses camming devices, and those are designed to press the rock harder than they're loaded. Usually, the coefficient of friction between rock and aluminium is less than one. This design also allows the cam to hold in slots that get wider towards the surface.

Thanks. To clarify, though, I do understand why one would be safe while the other wouldn't; what I was interested in was the actual technique a climber uses to determine that. In other words: there are certainly rocks for which both vertical and horizontal forces are safe, and rocks for which both vertical and horizontal forces are unsafe, but where and how is the line drawn to determine that vertical is safe while horizontal isn't? Is it just feeling and experience, or are there actual rules people generally follow?

There aren't any rules, but the feelings are usually strong ones. Consider the piece of rock in the article. It weighs tons, and has resisted gravity for centuries, so it would take horrible luck for it to fall off while someone was climbing it. On the other hand, no one would plan to fall on it - not even the guy who climbed it.

If the flake is 10m long it weighs many tons, adding 75kg of climber in the same gravity direction is negligible. Putting a torque on the rock along the breaking line is a different story.

(probably more like 50kg for skinny zero % body fat climber types)

The forces involved are considerably more than body weight in a lead fall (potentially tens of feet before you're caught; at one point in the article they mention the possibility of a 70 foot fall). 10 kN (roughly equivalent to 1000kg static hanging load) is a typical rating for lead climbing protection gear, and the only reason the force is that low is because the rope stretches considerably, reducing the max force exerted at any given instance.

The problem is that the type of protection you would typically use for this situation are spring loaded camming devices. These consist of logarithmic spirals that expand when they are pulled. If you fall on one of those in a flake, you are essentially taking a car jack to it.

http://en.wikipedia.org/wiki/Spring-loaded_camming_device

The fast'n'loose rule: if it is loose, don't hang on it or put gear behind it unless absolutely necessary!

That said, there's a bunch of well-known "expando" flakes, particularly in big-wall granite areas like Yosemite, that are necessary for certain routes.

Even the ones that have been around for a while, though, do eventually come down: http://www.rockymountainrescue.org/eldo_DB_27apr08.php

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